The correct answer is: A. can help reduce software development time
An operating system is a software that manages computer hardware and software resources and provides a platform for running application programs. It can help reduce software development time by providing a set of standard services that application programs can use, such as file management, memory management, and input/output. This can save developers time and effort, as they do not have to write code to implement these services themselves.
Option B is incorrect because an operating system does not necessarily provide the ability for other programmers to understand the software structure. This is a function of the programming language and the software development tools that are used.
Option C is incorrect because an operating system is not useless in most control applications. In fact, it is essential for many control applications, as it provides the platform for running the control software.
Option D is incorrect because only option A is correct.
Option E is incorrect because option A is correct.